This morning one of my customers could not receive inbound calls. They are running a Grandstream UCM6302 PBX with a Telnyx SIP trunk. The trunk uses registration, UDP for SIP, and the G722 and G711u (PCMU) codecs.
The Telnyx debugging report showed that they were receiving a “488 Not Acceptable Here” message from the PBX. A trace on the PBX itself showed the same thing, with a bit more clarity. Telnyx tried three times to connect the call, and the PBX rejected it three times:
But why was the call not acceptable?
One article I found suggested confirming in the trace that the RTP stream was using an acceptable codec. But there is no RTP stream in the trace; apparently the call never got that far.
Grandstream support suggested checking codecs and SRTP. Well I’d checked the codecs, and I was pretty sure I hadn’t changed the SRTP setting at Telnyx (SIP Connection > Inbound > Advanced):
And in the Grandstream trunk (Extension/Trunk > VoIP Trunks > Advanced Settings)?
Oops. The trunk is configured to require SRTP. Once I disabled that (don’t forget to click Apply Changes in the Grandstream UCM), inbound calls starting working again.
How did this happen? I was setting up SRTP on a different system last week. Apparently I chose the wrong browser tab and changed it for the customer.
TL;DR
If you get “488 Not Acceptable Here,” check that codecs and SRTP encryption match at both ends of the connection.